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The transport system in plants

treesOnly a Hobbit would be able to demonstrate that a plant’s transport system is a tree’s ability to walk to the Tower. In the real world, a plant’s transport system moves water and nutrients throughout the plant.

The main source of water and mineral salts is the soil in which the plant is rooted. Like a ring of bendable straws, the plant transports ions and water from its root hairs to the tips of branches and the ends of leaves. In ‘reverse’, the plant brings photosynthesis products from the leaves to the rest of the plant.

Unlike an animal, these are not real arteries, but rather masses of cells called xylem and phloem cells. And unlike the pumping heart of an animal, the liquid is moved by osmosis, the difference in concentration which water tries to equalize. They are found in roots, stems and leaves. Both xylem and phloem cells grow from cambium cells which are present in the stem.

Xylem cells transport upward from the roots only. This is demonstrated by unique colored carnations, which are soaked in colored ink. The cells line up like tubes which are wide and have thick walls. They bring ions and water to the tips of the plant, so the plant may grow branches and create buds, flowers, cones and fruit. These cells live about one year then die and need to be replaced.

Phloem cells create “sieve tubes” which can transport in both directions – water from the ground, and sugars (mostly sucrose) and nutrients such as amino acids which are created by photosynthesis. Their end walls have small holes with strands of cytoplasm running vertically through them. Companion cells alongside the phloem cells control the movement and direction. When the yearly ring of xylem cells dies in a tree, the phloem continues as sap. Since phloem transports in both directions, it has a lot of sugars from some trees such as the maple tree, which can be tapped to make maple syrup.Stem-cross-section2

The root hair cells of the root absorb water and mineral salts from the surrounding soil, which are transported throughout the plant to the root cap cells, which protect the growing tip of a stem or branch. The osmotic pressure between the xylem cells gives the plant some structure, so that the stems of flowers don’t wilt as long as there is water being passed along. In trees, these cells ‘line up’ as a ring; as the tree grows outward, the old xylem cells die, and new ones are created along the bark. This is why when people cut a ring around a tree below its bark, the tree will die – there is no completion of the transport system; nutrients cannot reach the roots or branches.

When the water and mineral salts reach the leaves, the water evaporates off the leaves, causing a constant pull upwards. This is called transpiration. The leaf can control the rate of evaporation by opening and closing its stomata (pores). If there is no water, the leaf shrivels and dies. Light stimulates the opening of the stomata. Other environmental factors also affect the rate of transpiration, such as temperature, humidity and even wind.

Deciduous plants are those which seem to hibernate in the winter. As the weather gets colder, transportation of water and nutrients slows down. Winds cause fast transpiration and sunlight is in low supply. Sugar gets stored; depending on the sugars being created by a tree, this causes the colorful fall foliage. When the leaves can no longer photosynthesize, they fall off, and the place where the leaf was is sealed off, as are the tips of the branches and stems. This keeps the water pressure steady but unmoving. As the spring sun warms the plant, these caps are dropped and growth resumes. Leaves grow and darken as they photosynthesize, flowers pop up and form nuts or fruit, and the cycle resumes.

By understanding how and why a plant circulates water, mineral salts, sugars and amino acids, people can understand how to care for the trees and plants around them, and spot the signs that there is a problem with the transport system.

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Fusarium wilt in fruit plants

A tobacco plant suffering from Fusarium wilt

A tobacco plant suffering from Fusarium wilt

The term  Fusarium wilt may be unique to most of us, but it is a very familiar term to farmers and agriculturalists. Fusarium  is a genus of fungi; some species are plant pathogens and some are opportunistic infectious agents of humans and other animals. There are actually over fifty different species, depending on the host plant infected. It is referred to as a soil pathogen because it is usually carried and passed by the soil.

Fusarium is found all around the world, infecting plants when conditions are opportune, such as stagnant water and over-moist soil with bad drainage. The actual pathogen is Fusarium oxysporum, which is further differentiated into different  forma specialis (sub-species). The symptoms on the infected plants are wilting, chlorosis (yellowing or blanching), necrosis (tissue death), premature leaf drop, browning of the vascular (stem) system, stunting and damping-off (decay and death due to excessive moisture). The most important, indicative and noticeable symptom is the actual vascular wilting. Older leaves start to droop, then the plant stops growing, starts yellowing, and eventually dies. It usually occurs between the blossom and fruit-forming stages.

Fusarium oxysporum can attack sweet potatoes, tomatoes, bananas, pears, pigeon peas, muskmelons, cantaloupe, tobacco, legumes, basil, beans, carnation, chrysanthemums, watermelon, peppers, cucurbits (a type of gourd) and cereals. It infects a healthy plant by mycelia or spores penetrating roots. After the plant dies the fungus invades all tissues, sporulates (forms spores), and continues to infect neighboring plants. F. oxysporum spreads fastest through soils that have high moisture and bad drainage. Fusarium can be in farming soil, or, indoors, in carpet and mattress dust, damp walls, humidifier pans and other locations of stagnant water. Some species’ spores are inhalant, which can affect animals, livestock and humans.

The concern for agriculturalists is that if one plant succumbs, the soil, as the vector, will eventually allow it to spread, ruining entire crops. And this condition cannot be resolved by crop rotation. Also, as with many pathogens, the fungus can become more virulent, thereby attacking plants considered resistant. Fusarium can live in the soil for long periods of time as well as through infected dead plant tissue. Infected farms need to do a total “clean up” of the crop area. This disease control would begin by removing all infected plants, improving soil conditions, using resistant plants where available, and applying soil and systemic fungicides on the infected soil. Other methods which should be utilized include flood fallowing and using clean seeds rather than seeds from previous crops. A biologic control has not been determined, since closed environments such as greenhouses do not simulate the open-field environments in which it is commonly found. For melons, there has been success in grafting a susceptible variety onto resistant root stock.

Not all news is bad:

“There is growing interest in using Fusarium wilt as a form of biological control. Certain pathogenic strains of F. oxysporum could be released to infect and control invasive weed species. This type of control (called a mycoherbicide) would be more targeted than herbicide applications, without the associated problems of chemical use. In addition. F. oxysporum may compete with other soil fungi that act as pathogens of important crops. Introducing specific strains of F. oxysporum that are not pathogenic (or non-infectious mutants of pathogens) to nearby crops could take nutrients from other potential disease-causing fungi”  (Snyder, W.C. and Hansen, H.N. 1940. The species concept in Fusarium. Amer. J. Bot. 27:64-67.)

Not only is it economically ruinous to lose entire crops of fruit to this ubiquitous fungus, but there is a chance of infecting animals and human beings, through eating the plants and fruits or by inhaling the spores. Exposure to Fusarium can also occur through the skin at damaged sites such as wounds and burns.

In human beings, the most common problem is allergy to molds and fungi. If a person is strong immunologically, the symptoms are minor and usually local. There may be other symptoms, but the condition is very treatable and survival rate of a systemic infection is 67%.

However, if a person is immunosuppressed as a result of transplants or excessive corticosteroid use, the consequences can be serious. If the patient has prolonged and profound neutropenia (a decrease in white blood cells called neutrophils) or a T-cell deficiency, or is under HSCT (Hematopoietic stem cell transplantation ) treatment, the symptoms can include cutaneous lesions, sinusitis, endophtalmitis (an inflammation of the inner eye), and/or pneumonia. Unfortunately, the prognosis for survival in compromised patients is 0% to 4% in patients with more than one compromising condition or persistent neutropenia. With only corticosteroid therapy, the survival rate climbs to 30%.

Treatments for infections vary according to the condition of the patient and the symptoms presented; there is no standard approach and a limited number of clinical trials available. The best treatment is prevention, and when a patient is at risk, extensive protective measures are taken.

In conclusion, Fusarium wilt is known but still omnipresent. It can cost farmers entire crops, inflate fruit prices, and even threaten wildlife and people.  Therefore it is not to be dismissed and efforts need to be made worldwide to prevent conditions which can spread the fungus.

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